Updating tables after using QSqlRelationalTableModel:setFilter()

[panosk]

I have a subclassed QSqlRelationalTableModel and I'm trying to update the database after applying a filter to hide empty rows. However, the queries I use inside my model no longer work correctly. It seems that, after the filter is applied, a temporary model is created with less rows. No matter what I tried, I can't make this work. The filter is applied in another class when I create the view.
The query copies all the text from a column in the referenced table to a column in the reference table. If I remove the filter, everything works perfectly. Here is a version of the function with the query:

void ProjectModel::copyAll() {
   this->setFilter(QString()); // Here I'm trying to reset the filter
   this->select();
   QSqlQuery query;
   query.prepare(QString("UPDATE %1 SET target = :text "
                       "WHERE sourceID = :id")
               .arg(referenceTable));

   this->database().transaction();

   for (auto i = 0; i < rowCount(); ++i) {
       QModelIndex sourceIndex = index(i,MyModel::SIdReference);
       QModelIndex targetIndex = index(i,MyModel::TText);

       if (sourceIndex.isValid()) {
           if (targetIndex.data().isNull()) {
               query.bindValue(":text",sourceIndex.data().toString());
               query.bindValue(":id",targetIndex.row()+1); // +1 = rowid in db
               if (!query.exec()) {
                   qDebug() << "Not updated: " << query.lastError().text();
                   break;
               }
           }
        }
   }
   this->database().commit();
   this->setFilter("sourceText != ''"); // Here I try to reapply the filter
   if (select()) {
       while(canFetchMore())
           fetchMore();
   }
}